Session B36: Electronic Properties of Nanowires

1D-1D Tunneling in Vertically Coupled Quantum Wires

We report tunneling measurements between two vertically coupled quantum wires in a GaAs/AlGaAs double quantum well structure with a 7.5 nm barrier. Split gates above and below the electron bilayer define each quantum wire and allow separately controlled 1D densities. Separate contacts are achieved with additional depletion gates. Parallel conductance as a function of split gate voltages provides a map of the 1D subband occupation; tunneling measurements can be made with any combination of subbands occupied in each wire. The full tunneling spectroscopy is measured using both a voltage between the wires and parallel magnetic field to explore the energy and momentum dependence of the tunneling. We observe a number of features, such as resonance peaks at high parallel magnetic fields, that can be explained within the framework of non- interacting 1D systems. These resonance features change in a systematic way as the number of occupied subbands changes. Other characteristics of the data such as very broad tunnel resonances as a function of the interwire voltage may require many-body interactions for a complete description of the tunneling physics. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04- 94AL85000.   

Tunable double quantum dots in InAs nanowires defined by local gate electrodes.

We present low-temperature transport measurements on quantum dots induced in homogeneous InAs quantum wires 50 nm in diameter. Quantum dots are induced by electrical depletion of the wire using local gate electrodes with down to 30 nm electrode spacing. This scheme has permitted the realization of fully gate-defined multiple quantum dots along the nanowire [1]. Tunability in double quantum dots is a prerequisite for the system to be operated as a quantum gate. We demonstrate control over the lead tunnel barrier transparencies and, in the case of double quantum dots, the interdot coupling. Using the local gate electrodes also as plunger gates we measure double dot honeycomb stability diagrams which show the transition from a single large dot to two weakly coupled dots at 4.2K. The induced quantum dots can be tuned into the few-electron regime which is shown from Coulomb blockade measurements. We extract values of orbital energy-level spacings, capacitances and capacitive and tunnel interdot coupling for this system. [1] C. Fasth et al., NanoLett 5, 1487 (2005).   

Orbital and Spin Effects in Single- and Double-Quantum Dots Defined in InAs/InP Nanowire Heterostructures.

Heterostructures in semiconducting nanowires are highly promising in terms of their potential for novel physics and device applications. We present measurements on single (double) quantum dots fabricated using InP double (triple) barrier heterostructures in InAs nanowires. Transport spectroscopy measurements on single dots show that we can design few-electron quantum dots in nanowires where the well defined geometry leads to shell structure effects. We also show that the Zeeman splitting of the first few electrons can be engineered to any desired effective g-factor between $\vert $g*$\vert $=2.5 and values close to the bulk value of InAs $\vert $g*$\vert $=13 [1]. In extension of this, transport spectroscopy in the few- electron regime of double quantum dots consisting of two InAs islands in series have been performed. These double dots are tuned using a single homogenous backgate, which together with a source-drain bias allows us to controllably empty both dots down to the last electron. Again we observe a shell-structure which can be linked to the geometric cross-section of the wire and an orbital blockade effect is observed depending on the character of the wavefunctions in each dot. [1] M. Bj\"{o}rk et al., cond-mat/0507433, 2005   

Ultra-dense nanowire arrays

The development of the superlattice nanowire pattern transfer (SNAP) technique has allowed for the fabrication of highly ordered arrays of hundreds of nanowires (both metallic and semiconducting) at pitches down to 16 nm and aspect ratios up to 10$^{6}$. Applications of these nanowire arrays range from bridging length scales via binary-tree demultiplexing [1], to the development of ultra-dense arrays of molecular switch tunnel junctions ($\sim $ 1 TBit/in$^{2})$, to the integration of complementary logic arrays within a crossbar architecture. In addition, at the narrowest pitches the periodicity of the SNAP array is only a few tens of atoms, allowing access to length scales compatible with coherent electronic transport and opening the door to fundamental studies. These topics will be discussed within the context of the flexibility of the SNAP fabrication technique and its wide applicability to a number of both basic and applied challenges in nanoscience/nanotechnology. [1] Science, 310, 465 (2005).   

Ag nanowires on Cu(110) and Ni(110): atomic structure and electronic dimensionality

Epitaxial Ag nanowires have been found to self-assemble on Cu(110) or on Ni(110) at Ag coverages exceeding 1.2 ML. The atomic and electronic structure of these nanowires have been characterized by scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES). STM shows that the Ag(110)-oriented nanowires that are straight and parallel to the [110] direction with a uniform width (height) ranging between 10-30 nm (2-3 nm) depending on coverage. The nanowires have a triangular cross-section and expose two sides of facets that slope at an angle of approx 25 deg with respect to the surface. Overall length-to-width aspect ratios up to 20:1 have been observed. The substrate-wire lattice mismatch induces an anisotropy in the nanowire with a strained lattice match in the [001] (across-wire) direction and an incommensurate periodicity, essentially decoupled from the substrate, in the [110] (along-wire) direction. Angle-resolved photoemission data reveals Ag d-band dispersion in the vertical (or (110)) and the lateral [110] (or along-wire) direction, but absence of dispersion in the lateral [001] (or across-wire) direction because of the limited dimension of the nanowire width. This anisotropic electronic dimensionality correlates with the structural asymmetry.   

Electronic structure of self-assembled Si nanowires on Ag(110) surfaces

Much attention has recently been paid to the physics of one- dimensional (1-D) systems, since exotic properties are predicted from basic theoretical consideration. However, it is not easy to realize such one-dimensional systems experimentally. Recently, Leandri et al.[1] have reported the growth of self-assembled 1D linear structures of silicon on Ag(110), perfectly aligned along [-110] direction and 16$\AA$ wide. We have investigated the electronic structure of such self-assembled Si “nanowires” using angle resolved photoemission spectroscopy. The Si 2p core level line shows two narrow components in agreement with earlier work [1]. Silicon-induced features in the valence band region are observed, most clearly within the band gap of the Ag s-p states. The silicon-induced band exhibits a sizeable dispersion only in the direction along the linear structure, i.e. the [- 110] azimuth of the Ag(110) substrate. Details of the experimentally observed bands will be presented and related to the atomic structure within the 1D structure and its arrangement on the silver substrate. T.O. acknowledges financial support from Max Planck Society. Experiments were performed at the Advanced Light Source, Lawrence Berkeley National Laboratory operated by the U.S. DOE under Contract No. DE-AC03-76SF00098. [1] C. Leandri et al., Self-aligned silicon quantum wires on Ag (110), Surface Science 574 (2005) L9–L15   

One-dimensional Pt induced chains on Si(337)

The use of high index Si surfaces as templates for the formation of adsorbate induced one-dimensional chain structures have attracted considerable interest. These systems have been used as a test bed in which to study low-dimension physics and components of nanoelectronics. In addition to the Ag and Au induced chains reported to form on the Si(337) surface, Pt also produces one-dimensional chains. Angle-resolved photoemission spectroscopy was used to investigate the electronic structure of these new Pt chains. The valence band mapping confirms the one-dimensional nature of these chains as seen in LEED. Supported by ONR and DOE.   

The Coulomb blockade regime in disordered AlAs cleaved-edge overgrown quantum wires

We present transport measurements on AlAs quantum wires in the pinch-off regime, where the wire breaks up into zero-dimensional islands of electrons and Coulomb blockade sets in. Conductance resonances as a function of gate bias vanish at low temperature T, and a gap bounded by two peaks opens up in the differential conductance as a function of source-drain dc bias V. We propose two interpretations for these results: (i) low T, low V transport is limited by incoherent tunneling through 2 asymmetric quantum dots in series; (ii) at pinchoff the wire carries a collective mode that is pinned by disorder, but can be depinned by a finite threshold bias V.   

The transition from a one-dimensional to a quasi-one-dimensional state in interacting quantum wires

Upon increasing the density of electrons in a quantum wire, the system undergoes a transition from a one-dimensional to a \textit{quasi}-one-dimensional state. In the absence of interactions between electrons, this corresponds to filling up the second subband of transverse quantization. On the other hand, strongly interacting one-dimensional electrons form a Wigner crystal, and the transition corresponds to it splitting into two chains (zig-zag crystal). While the two subbands in the non-interacting case represent two gapless electron modes, in the Wigner crystal the two chains are locked, i.e., the relative motion is gapped, and only one gapless mode remains. We study the evolution of the system as the interaction strength changes. In particular, we establish that only one gapless mode exists near the transition at any interaction strength.   

Exciton Energy Transfer between Asymmetric Quantum Wires

We present theoretical result and data for the Stokes exciton transfer rate from a narrow quantum wire (n-QWR) to a parallel wide QWR (w-QWR) separated by a wide barrier and also to an array of parallel w-QWRs. The transfer rate is calculated as a function of the distance $d$ between n-QWR and w-QWR and also the array. The dependence of the rate on the temperature and the localization radius is studied for free and localized excitons, respectively. Both the resonant and non-resonant rates are considered. We find that, for energy transfer between two QWRs, the F\"{o}rster dipole-dipole transfer dominates the transfer rate at short and intermediate distances. The photon-exchange transfer prevails only at an extremely long distance where the rate is negligibly small. This behavior is in contrast with the two-dimensional quantum wells, where the photon-exchange mechanism is dominant except at a very short distance. However, the photon-exchange transfer rate continues to increase as the array size grows to a macroscopic scale due to its slow range dependence while the dipolar rate saturates quickly with the array size. The prediction of the theory is consistent with the data from V-groove GaAs/Al$_x$Ga$_{1-x}$As double QWRs. Supported by the US DOE (SKL), Swedish Foundation for Strategic Research, Swedish Research Council, and Ericsson's Research Foundation.   

Spin excitation velocities in multi-channel quantum wires

We investigate how an external magnetic field affects the interaction corrections to the velocities of spin collective modes in quantum wires. We show that both many-body and single- particle effects are important. The former generate logarithmic field dependence that can alter the Hartree-Fock results by up to a factor of two. On a single-particle level, the field modifies the subband wavefunctions and therefore the effective interaction strength. This theory is applied to quantum wires fabricated by the recently introduced cleaved edge overgrowth technique and the results are compared with experiments [Auslaender \emph{et al.} Science \textbf{308}, 88 (2005)].   

Junctions of Three Quantum Wires of spin-1/2 Electron System

We study a junction of three interacting quantum wires which are joined by a ring enclosing a magnetic flux. The wires are modeled as single channel spin-1/2 Tomonaga-Luttinger Liquids (TLL). We study the problem by identifying the boundary conditions corresponding to the low energy fixed points through both Delay Evaluation Boundary Condition and Boundary Conformal Field Theory method. We obtain a rich phase diagram as the function of the TLL interacting parameter, $g_c$ and $g_s$, including a chiral regime in which the asymmetric current flow is highly sensitive to the sign of the flux and a phase in which electron pair tunneling dominates.   

Effect of electron-electron interaction on linear and nonlinear optical phenomena in quantum dot systems in interlevel resonance region

We theoretically investigate linear and nonlinear optical phenomena in quantum dot (QD) systems caused by interlevel transitions. Effect of electron-electron (e-e) interaction in the systems on the optical phenomena is in the focus of the work. The e-e interaction is taken into account by employing the self-consistent field approach in the quasistatic limit. We show that presence of metal surface, and especially another resonant system, can dramatically enhance the effect of the e-e interaction on the optical phenomena. We discuss the conditions for the intrinsic optical bistability in QD systems caused by the e-e interaction. The obtained results can find applications for design, fabrication, and exploiting nanooptoelectronics devices, in part, all-optical components like QD-based optical switches and optical transistors.